A lot of people dislike multiboxers. Surprisingly, the same people think that multiboxers are some kind of elite, like a mix of advanced sysadmin and a great pianist. It is not true. Multiboxing is all-around 2 macros and 1 keybind!
The first macro is following:
It makes minions run for main obediently.
The second macro is assisting:
It makes minions pick up the main's target.
Most probably, you are already familiar with these macros. The third thing likely will be new. It is keybind "interact with target" (IWT).
This keybind does the same as right-clicking. It opens a dialog with NPC, attacks target, etc. IWT is not assigned by default, so casual players don't use it usually. But multiboxers have to use it frequently.
The usability of IWT can be increased by enabling the "click to move" option in WoW settings. If the character is out of interaction range, it will run to target and only then starts interacting.
Damage spells should be bound to the same key, so a single keypress causes all characters to fire. The most useful key for this is the mouse wheel - you can do damage by constantly spinning it.
And this is all! These 5 things are enough to start multiboxing. You bind minions to your main character by following. Then you give them a target by assist and begin damaging. Melee characters can be sent into the attack by IWT. IWT also used to loot dead targets... Profit! WoW is surprisingly suitable for multiboxing.
But, you may say, what about software? It is very complicated and needs a long setup! Well, it depends. If you want to run actual 10 ppl dungeons only by yourself - yes. This needs scripting languages like AutoHotKey or HotKeyNet and requires at least basic programming skills. Or you must use professional multiboxing software like IsBoxer, which is paid and requires setup. However, if all you want is leveling and farming, you can download one of the simple free broadcasters like Levelbuddy.
But multiboxing also requires a second account, isn't it? No, it is not. You can play up to 8 characters on the same account simultaneously. All you need is another WoW subscription, which is free until 20 lvl.
So, please, stop thinking that multiboxers are Atlas and Titans of WoW. They are just unusual players who use some WoW features (which make multiboxing so easy) for fun and profit.
Now you can make your own judgment.
Multiboxing is a force multiplier. It increases all kinds of things - firepower, self healing, complexity... I played around with it on a private server before the Classic launch. The general idea of commanding all your followers around with macros works, but is clunky. And it's not like there's a better way of doing it either, it's just a clunky process.
During the scarab lord grind, there were plenty of horde players with 5, 10, 20 accounts (I'm just going to call a subscription an 'account' here) nuking people to clear hives. These multiboxers were pretty much able to delete enemy players at will. But they were also extremely susceptible to AOE attacks and fears. The process of having all the followers running around with the /follow command means that a well placed fear will make the multiboxer's characters all scatter in random directions and generally desync. It takes some effort to round them all back up in a single place, and this kind of chaos makes them pretty vulnerable to coordinated counter attacks. It's still just a single player behind all the characters there, so that person can become overwhelmed pretty easily when confronted by a handful of other people. Even a small group of other players is capable of faster inputs on aggregate than the multiboxer, so they can get blown up with stuns/grenades/sappers and pushed around.
I got no problem with multiboxers. It's still a player at the helm of them all. They're definitely not invincible and have their weaknesses.
I'm leveling a shaman right now (better late than never) to dualbox with my warrior main and do jump runs, hopefully I got it done before phase 6.
The only macro I use is /follow (character name) and bind it to mousewheel, having two monitors allows you to just move the mouse to the shaman screen and just mousewheel to follow without cancelling actions or alt tabbing with your main.
I'm thinking about binding Windfury totem to Mousewheel down, and Healing wave + follow on Mousewheel Up.